Gemara

113a (Mishnah): If Reuven lent Shimon, he may Memashken him (take a security from Shimon's house) only through Beis Din.

He may not enter Shimon's house to take it - "ba'Chutz Ta'amod".

115a (Beraisa): "Lo Savo El Beiso La'avot Avoto" - you may not enter the borrower's house, but you may enter the house of the Arev;

"Lekach Bigdo Ki Arev Zar" (the lender may take the borrower's garment);

Alternatively: you may not enter a borrower's house, but you may enter the house of one who owes for your labor, for rental of your animal or (inn) room.

Suggestion: Perhaps you may enter even if this was converted to a loan (e.g. a payment date was set)!

Rejection "Mashas Me'umah (any debt)".

Bava Kama 27b (Rav Yehudah): One may not take the law into his own hands;

(Rav Nachman): One may do so.

All permit when needed to avoid a loss. They argue about when there is no loss. Rav Yehudah forbids, for he can go to Beis Din. Rav Nachman permits. Since he acts properly, he need not exert himself to go to Beis Din.

(Beraisa - Ben Bag Bag): Do not enter your friend's property to take what is yours without permission, lest you appear like a thief to him! Rather, blunt his teeth and say 'I am taking what is mine!'

Berachos 5b: Four hundred barrels of Rav Huna's wine spoiled. Rabanan suggested that it is because he did not give to his sharecropper his share of vine branches.

Rav Huna: He stole much more than that from me!

Rabanan: This is like people say, that one who steals from a thief tastes theft.

Rishonim

Rif (Bava Kama 12b): In monetary laws, the Halachah follows Rav Nachman, who says here that one may take the law into his own hands.

Rambam (Hilchos Sanhedrin 2:12): One may take the law into his own hands if he has the power.

Rosh (Bava Kama 3:3): One may take the law into his own hands only if the other comes to steal from him, or is holding his item. He may not (forcibly) take a security for an existing debt. That must be done in Beis Din (113a). Therefore, if he hit him, he is liable like anyone who hit him.

Rivash (396): The Torah forbids a lender to enter the borrower's house to take a security. If Ploni seeks to damage his property, he may harm Ploni to stop him. One may seize Ploni's property in others' Reshus to be a security for what Ploni owes him, for this is taking the law into his own hands. If the debt is not a loan, he may even enter Ploni's house. However, if he seized for a loan, b'Di'eved it helps. R. Chananel allows one who has a loan or deposit from Ploni to keep it for what Ploni has of his, for one may take the law into his own hands. However, he must go to Beis Din and say what he has of Ploni and what Ploni has of his. He is believed up to the value of what he holds. Even though he goes to Beis Din, R. Chananel needed to say that one may take the law into his own hands, to permit keeping it for a security. This is only to take his own, e.g. what was stolen from him or what he deposited, like Ben Bag Bag says.

Poskim

Shulchan Aruch (CM 4:1): One may not take the law into his own hands to take a security for a debt.

Rema: The reason is explained in Siman 97:6. Some say that this is only for a debt, but if he is owed not for a loan, or if he need not take a security because he already has a deposit from him, or it is in another's hands, one may seize it.

SMA (3): Why does the Rema say that some say so? One may take a security from an Arev (guarantor), or from a renter for the rental (CM 97:14)! It seems that the text of the Rema begins 'one may take the law into his own hands... ', i.e. even to hit. He continues to say that some say that one may always seize...

Rema: Some say that taking the law into his own hands is only when he hurt him; one may do so only if he can prove that it is his. However, one may always seize property for security, and then go to Din with him.

Source (Gra 15): A Beraisa permits entering the house of the Arev to take a security, or the house of one who owes for your labor or for rental. Rav Huna was punished because he did not intend to go to Din with his sharecropper.

Maharik (161): Even though the Halachah follows Rav Nachman, this is only when one retrieves his item. Surely, he does not argue with the Mishnah that forbids a lender to take a security . The verse explicitly forbids entering his house for this! Surely a Tzibur may not take the law into its own hands without clarity (about what is owed). Taking the law into his own hands is to act like a judge, to collect property in place of other property, or to beat someone or make him lose until he pays. Obviously, one may seize his item. Seizing property is merely called 'Tofes.' A borrower may repay a loan with anything, even bran, therefore the lender may not seize. Rav Huna was rebuked for taking something else in place of what was taken from him. It seems that Rabanan did not know of the theft, and nevertheless had he taken his own, he would not have been punished. One may not use Nochrim to recover his own, unless he can prove that it is his. If not, everyone will grab another's Talis through Nochrim, and say 'it is clear to me that it is mine.'

Rebuttal #1 (Yam Shel Shlomo 3:5): We cannot say that anyone may seize and say 'it is mine', and be believed with a Vadai claim. If so, the most powerful person will seize everything! The Maharik himself forbids seizure through Nochrim. It is easier and more common to seize by oneself! Hagahos Maimoniyos requires one to be sure that it is his item even to enter the other's house. The Maharik requires this only to take something else in place of it, or to cause a loss. Sefer Yere'im says that had Rav Huna calculated what his sharecropper owed him, it would be like a loan, and even to take the stolen object would be taking a security, which is forbidden

Rebuttal #2 (Bach DH Kosav): Seizure helps only if he received the object b'Heter, or if it was not in front of witnesses, so he has a Migo.

Defense (Tokfo Kohen 114-117): The Maharik permits seizing when he knows that it is his item, just he is unsure if Beis Din will accept his proof, e.g. (in the case he ruled about) there was a claim against the document. The Maharik never meant to allow seizing when the other side denies. Even seizure b'Heter helps only with a Migo.